Electrically conductive pastes composed of a base metal such as copper or nickel and an inorganic binder dispersed in an organic binder are widely used as terminal electrode materials for multilayer ceramic capacitors. These conductive pastes must be electrically conductive after firing and are also required to have such properties as good adhesion to the capacitor assembly before and after a firing treatment. For example, U.S. Pat. No. 3,922,387 discloses a composition comprising noble metals with a nickel oxide addition to improve adhesion.
A number of related problems have arisen with the growth over the past few years in the use of high-density surface mount technology. For instance, a relatively high temperature of 800 to 900° C. is needed to fire present conductive pastes. When the terminal electrodes on a capacitor are formed by firing within the above temperature range, this process creates large internal stresses within the capacitor because of diffusion of the metal constituents within the paste to the capacitor's internal electrode, sintering shrinkage at junctions between the terminal electrodes and the capacitor assembly (e.g., ceramic), and diffusion of the inorganic binder components in the paste through the capacitor assembly. The result is problems such as crack formation in the capacitor assembly due to rapid temperature changes that take place during soldering of the chip capacitor to the circuit board or during solder repair and crack formation in the capacitor when the substrate warps due to the external application of bending forces thereto.
As noted, because the electrically conductive pastes have a relatively high firing temperature when used in terminal electrodes, they have a low thermal shock resistance during soldering and related processes. Moreover, they have an inadequate strength to warping or bending of the circuit board on which the chip capacitors are mounted (i.e., flexural strength). As a result, cracking readily arises due to internal stresses within the capacitor assembly during firing, which causes low reliability. When the conductive pastes are fired at relatively low temperatures of 650 to 780° C., the flexural strength improves, but the density is inadequate and other characteristics of the chip capacitor decline, making it impossible to achieve good reliability.
It is therefore the object of the present invention to provide novel compositions which, when fired in a relatively low temperature range of 700 to 770° C., are capable of forming terminal electrodes that satisfy the various characteristics desired for use in multilayer ceramic capacitors, and that have a high mechanical strength and a good reliability such as thermal impact resistance.
In addition, the presence of a metal oxide in the inventive composition enables a sintered film of suitable density to be obtained at a low firing temperature of 700 to 770° C. Moreover, the density of the sintered film does not change much within a firing temperature range of 700 to 770° C., thereby ensuring a broad process window.